This invention pertains to apparatus for metering the flow of a liquid through a conduit by introducing a gas bubble into the liquid stream and timing the passage of the gas bubble between two points located a predetermined distance apart along the conduit. Such flow meters have found application in liquid chromatographs, fuel systems, laboratory instruments, and medical devices where small liquid flows on the order of one milliliter per minute are typically encountered. Devices employing related principles have been proposed for measuring small liquid volumes in similar applications.
An important criterion for the proper operation of such flow meters is that the gas bubble introduced into the conduit should have a diameter substantially equal to the inside diameter of the conduit, thus completely dividing the liquid into two segments separated by the gas bubble. If the diameter of the gas bubble is less than the inside diameter of the conduit, the bubble may not flow through the conduit at the same rate as the liquid and may furthermore become attached to the wall of the conduit, requiring the user to manually flush the apparatus to release the bubble. Consequently, a gas pressure must be developed which is great enough to introduce a gas bubble of sufficient volume into the conduit against the opposing pressure of the liquid. This requirement is complicated by the fact that the pressure of the liquid within the conduit may be substantially different from the pressure of the surrounding atmosphere due to flow restrictions, changes in elevation of the liquid, and deliberately applied pressures or vacuums, some or all of which may be encountered by the liquid downstream of the flow meter.
Mechanisms for introducing the gas bubble into flow meters of the prior art have included manually-operated syringes, stop cocks, and three-way valves of predetermined interior volume; gas pumps; fluidic amplifiers; and electrodes for producing gas by electrolysis. Manually-operated syringes can develop sufficient gas pressure but require substantial operator skill when injecting a predetermined volume of gas into the conduit and are furthermore unsuited to applications requiring fully-automated metering. Stop cocks and three-way valves similar to the valve described by Gildner in U.S. Pat. No. 3,693,436 may not introduce a gas bubble of sufficient volume if the pressure of the liquid in the conduit equals or exceeds the gas pressure within the supply tank to which the valve is connected. On the other hand, active mechanisms such as gas pumps and fluidic amplifiers may readily produce a gas pressure substantially greater than that of the liquid, causing the gas bubble to disintegrate into a number of smaller segments upon introduction into the conduit. Approaches relying on electrolysis of the liquid to produce gas, such as the in method disclosed by Prestele in U.S. Pat. No. 4,559,831, are limited to very low flow rates through small diameter conduits, and are applicable only to electrolytic fluids. Although these devices may be adapted to operate at a specific liquid pressure, the applicants are unaware of any transit time flow meter of the prior art which introduces a gas bubble of predetermined volume into the conduit independently of the pressure of the liquid in the conduit.
Once a gas bubble of sufficient volume has been introduced into the conduit, the time required for the gas bubble to transit between two positions located a predetermined distance apart along the conduit is measured and may either be displayed directly or may be used to derive an indication of the flow of the liquid through the conduit. Several prior art flow meters have incorporated an optically transparent conduit with two or more bubble detectors located outside the conduit, each detector comprising a light emitting device and an opposing light sensing device which detects the change in optical transmittance between light emitter and light sensor resulting from the passage of the trailing interface between the gas bubble and the liquid. Prestele, in U.S. Pat. No. 4,559,831, discloses apparatus in which the bubble is sensed by a pair of spaced electrodes, the electrical resistance between the electrodes being higher if the bubble is between the electrodes and lower otherwise. Prior art flow meters which employ only a limited number of such bubble detectors or electrodes positioned at considerable intervals along the conduit do not provide any means for producing an image representative of the gas bubble and the liquid within the conduit. Prior art flow meters which lack such conduit imaging means are limited in their ability to control the volume of the gas injected into the conduit through direct observation of the gas in the conduit. Similarly, such flow meters are limited in their ability to detect disintegration of the gas bubble arising from irregular or pulsatile liquid flow when the gas bubble is in between bubble detectors. Flow meters which do not incorporate conduit imaging means may furthermore be sensitive to small, artifactual gas bubbles suspended within the liquid and carried by the flow into the conduit. Another limitation of such flow meters is that in order to qualitatively observe the flow of the liquid, the user must physically view the conduit and the gas bubble within it, which requires that the user be at the location of the conduit.
Flow meters which employ the transit time of a gas bubble between two locations have traditionally been used in liquid chromatographs and other laboratory instrumentation in which the flows are substantially constant as maintained by a laboratory-quality pumping device. However, in other pump metering applications, restrictions to the flow of the liquid and pressures applied to the liquid from other sources downstream may, depending on the magnitude and effect of these pressures on the pump, cause the flow to be highly varied. The mechanism of the pump may also produce a preset flow by dispensing the liquid in rapidly-delivered boluses, each bolus having a small, predetermined volume and the time between each bolus delivery thereby determining the flow. Flow meters of the prior art as described may react to such flow conditions by producing erratic indications which are difficult for the user to interpret. Such flow meters can be made to average irregular flows by employing a conduit of larger volume. However, this proportionally increases the measurement time and reduces responsivity of the flow meter to smaller flow perturbations under more continuous flow conditions.
To overcome the some of the limitations of prior art devices, an object of the present invention is to provide apparatus for metering liquid flow which is based upon the transit time of a gas bubble between at least two locations along a conduit, incorporating a pressure sensor which produces a signal proportional to the pressure of the liquid in the conduit, and a pressure generator which responds proportionally to the signal to supply gas at a predetermined pressure to a gas injector. The predetermined pressure is sufficient to allow the gas injector to inject a predetermined volume of gas into the conduit to form a gas bubble which always separates the liquid into two segments, largely independent of the absolute pressure of the liquid in the conduit.
A further object of the invention is to provide apparatus for metering liquid flow, including an imaging sensor array for producing a conduit image representative of the gas and the liquid in the conduit of a bubble transit flow meter, the conduit image comprising a number of elements which form an image of the liquid and at least one element which forms an image of the gas bubble. The apparatus also includes an image processor which analyzes the conduit image to control the introduction of gas into the conduit. Related objects of the invention provide means for injecting a predetermined volume of gas into the conduit, and means for injecting a second gas bubble when a first gas bubble introduced into the conduit is disrupted by irregular flow, or if small gas artifacts are detected within the conduit. A further related object of the invention provides means for displaying the conduit image on an array of visible indicators, thereby allowing the user to qualitatively observe the movement of the gas bubble within the conduit without having to physically view the conduit.
Another object of the invention is to provide apparatus for accurately metering irregular liquid flow, including an imaging sensor array for producing a conduit image as above, a timer for producing a number of transit times obtained from the conduit image, and a transit time analyzer for processing the transit times produced by the timer. A related object provides an algorithm executed by the time analyzer which variably and automatically averages the transit times depending on the variance of the transit time data. Advantageously, this algorithm does not substantially increase the measurement time or reduce responsivity of the apparatus to smaller flow perturbations under more continuous flow conditions.
The applicants are aware of the following United States Patents which are more or less relevant to the subject matter of the applicants' invention.
______________________________________ 3,693,436 09/1972 Gildner 73/194 E 4,559,831 12/1985 Prestele 73/861.05 3,403,555 10/1968 Versaci et al. 73/194 4,210,809 07/1980 Pelavin 250/343 3,688,574 09/1972 Arutunian et al. 73/194 E 3,621,715 11/1971 Soderkvist et al. 73/194 E 3,739,636 06/1973 Versaci et al. 73/194 E ______________________________________